Code Division Multiple Access (CDMA) has been widely used in satellite communication field such as GNSS. Civic GPS (Global Positioning System) application is rapidly developed. To achieve excellent GPS navigation, sufficiently available satellite number and qualified signal measurement accuracy are basic requirements to be satisfied. However, such conditions are hardly satisfied in deep urban circumstances.
A GPS receiver suffers problem when receiving signals in urban. For example, signal power level may abruptly and rapidly change since the receiver, which may be set up in a vehicle, is moving, or the signal is blocked by buildings.
The receiver uses coherent integration and non-coherent integration to obtain sufficient signal power. Conventionally, a coherent integration period is fixed and is ranged from 1 ms to 20 ms, in general. In addition, a non-coherent number is also set in advance. If the coherent integration period is predetermined to be 20 ms, the receiver integrates the receiver power for 20 ms. The 20 ms coherent integration is stored, and another 20 ms coherent integration is calculated. The 20 ms coherent integrations are accumulated. The non-coherent number of coherent integrations are accumulated. This is called non-coherent integration. A non-coherent integration period is predetermined since the non-coherent number is fixed. For example, the non-coherent integration period is predetermined as 2 seconds. Then, the 20 ms coherent integrations are accumulated until a 2-second non-coherent integration is obtained. The non-coherent integrated signal power is checked to see if it is sufficiently high. However, in bad circumstances, such as urban, the signal may be sometimes very weak or even be blocked. It will take a significantly long time to obtain sufficient signal power level by using the prior art scheme. For example, when a receiver receives a satellite signal in urban environment, the power of the satellite signal may be changed dramatically. The strong signal power may suddenly become weak. Under such a circumstance, the receiver is likely to lose lock for the satellite if the integration time is not long enough. Therefore, it is necessary to use a long integration interval. However, the weak signal power may also suddenly become strong. If the long integration time interval is still used, the response of the receiver to the satellite signal will become slow and blunt. To overcome such a problem, the integration period needs to be more effectively and adaptively controlled.